Hyaluronic acid ("HA") is a naturally-occurring linear polysaccharide with repeating disaccharide units composed of gluconic acid and N-acetyl-glucosamine. As used herein, the term ,"HA," means hyaluronic acid and any of its hyaluronate salts.
HA is widely distributed in animal tissues; it is present in high concentrations in synovial fluid and the vitreous body of the eye, and in connective tissues of rooster comb, umbilical cord, and dermis. The molecular weight of hyaluronic acid isolated from natural sources generally falls within the range of about 6.times.10.sup.4 to about 1.2.times.10.sup.7 daltons. When implanted or injected into a living body, naturally-occurring HA has excellent biocompatibility and does not cause a foreign body reaction.
When used as a medical product, HA generally must be sterilized. A common method of sterilization is chemical sterilization. Chemical sterilization, however, can cause chemical contaminants to remain in the HA, thereby potentially limiting the utility of sterilized material.
Dry heating is another conventional method for sterilizing HA. Unlike chemical sterilization, dry heating results in little if any chemical contamination. Nonetheless, dry heating of HA degrades the HA by dramatically reducing its molecular weight and changes its conformational structure.
HA has also been sterilized in solution by autoclaving. Autoclaving of HA solution degrades the HA to a lesser extent, as measured by a decrease in molecular weight, than does dry heating. Nonetheless, the molecular weight loss remains significant; and the utility of the sterilized HA is thereby limited.
Therefore, a need exists for a method of sterilizing HA which overcomes or minimizes the above-mentioned problems.